BACKGROUND OF THE INVENTION
[Technical Field]
[0001] The present invention relates to an initial core of a nuclear reactor and a method
of loading fuel assemblies of a nuclear reactor and more particularly to an initial
core of a nuclear reactor and a method of loading fuel assemblies of a nuclear reactor
suitable for application to a boiling water reactor.
[Background Art]
[0002] In a boiling water reactor, a plurality of fuel assemblies are loaded in a core disposed
in a reactor pressure vessel. These fuel assemblies include a plurality of fuel rods
filled with a plurality of fuel pellets manufactured with a nuclear fuel material
including uranium, a lower tie plate for supporting lower end portions of these fuel
rods, an upper tie plate for holding upper end portions of the fuel rods, and a channel
box of a square cylinder attached to the upper tie plate and extended toward the lower
tie plate. A plurality of fuel rods are bundled by fuel spacers for holding the mutual
intervals at a predetermined width and are arranged in the channel box.
[0003] A core installed in a reactor pressure vessel of a newly-built boiling water reactor
is called an initial core and all the fuel assemblies loaded in the initial core are
fresh fuel assemblies with a burnup of 0 GWd/t. In the boiling water reactor having
the initial core, a part of the fuel assemblies in the initial core is taken out after
end of the operation in the first cycle and is replaced with the fresh fuel assemblies.
A plurality of fuel assemblies taken out from the core after end of the operation
in the first cycle, have a lower enrichment than the mean enrichment of all the fuel
assemblies loaded in the initial core at the point of time of loading in the initial
core.
[0004] The boiling water reactor having the initial core must continue the operation without
supplying fuel assemblies over one operation cycle (for example, one year), so that
the initial core includes a fissional material in a quantity larger than the quantity
necessary to maintain the critical state. Therefore, the initial core holds excess
reactivity, and in order to control the excess reactivity, the boiling water reactor
is provided with a plurality of control rods, and furthermore, burnable poison is
mixed in the nuclear fuel material in the nuclear fuel rods included in the fuel assemblies
loaded in the initial core.
[0005] An example of such an initial core is described in Japanese Patent Laid-Open No.
2008-145359. In the initial core described in Japanese Patent Laid-Open No.
2008-145359, the quantity of the fissional material of the plurality of fuel assemblies arranged
in peripheral portion in the initial core is larger than the quantity of the fuel
assemblies arranged in the region on the inner side of the core from the peripheral
portion. In the region on the inner side from the peripheral portion, a plurality
of control cells including four fuel assemblies having a low mean enrichment are arranged
and the control rods for reactor power adjustment are inserted between the four fuel
assemblies composing the control cells.
[0006] Also in Japanese Patent No.
2550381, an initial core is described. In the initial core, in first cycle, no fuel assemblies
are arranged in a peripheral portion of the initial core and in the second cycle,
a plurality of fuel assemblies are arranged in the peripheral portion. As mentioned
above, no fuel assemblies are arranged in the peripheral portion of the initial core
in the first cycle, so that the number of spent fuel assemblies taken out from the
core after end of the first cycle can be reduced and the fuel cycle cost of the initial
fuel can be reduced.
[Citation List]
[Patent Literature]
[0007]
[Patent Literature 1] Japanese Patent Laid-Open No. 2008-145359
[Patent Literature 2] Japanese Patent No. 2550381
SUMMARY OF THE INVENTION
[Technical Problem]
[0008] In the initial core described in Japanese Patent Laid-Open No.
2008-145359, the quantity of the fissional material in a plurality of fuel assemblies loaded
in the peripheral portion in the initial core is increased and as a consequence, the
mean enrichment of the initial core is increased. If the mean enrichment of the initial
core is increased, the excess reactivity of the initial core is increased, so that
the excess reactivity of the initial core must be controlled by the increase in the
additional quantity of the burnable poison and the insertion amount of the control
rods. The profitability of the reactor is lowered due to the increase in the additional
quantity of the burnable poison and the exchange number of the control rods in the
periodic inspection is increased due to the increase in the insertion amount of the
control rods in the initial core.
[0009] In Japanese Patent No.
2550381, in the first cycle, no fuel assemblies are arranged in the peripheral portion in
the initial core, and in the second cycle, a plurality of fuel assemblies are arranged
in the peripheral portion, and thus the fuel cycle cost of the initial fuel loaded
in the initial core is reduced. The inventors followed this technical thought and
aimed at simplification of the operation of the control rods in the initial core.
[0010] An object of the present invention is to provide an initial core of a nuclear reactor
and a method of loading fuel assemblies of a nuclear reactor capable of simplifying
control rod operation.
[Solution to Problem]
[0011] A feature of the present invention for accomplishing the above object is an initial
core of a nuclear reactor comprising a central region disposing a plurality of first
fuel supports for supporting fuel assemblies in which a first cooling water supply
passage is formed for every fuel assembly supported and introduces cooling water to
the fuel assembly inserted in the first cooling water supply passage; and a peripheral
region surrounding the central region, and disposing a plurality of second fuel supports
for supporting fuel assemblies in which a second cooling water supply passage having
a pressure loss smaller than that of the first cooling water supply passage is formed
for every fuel assembly supported and introduces cooling water to the fuel assembly
inserted in the second cooling water supply passage;
wherein a plurality of water regions with no fuel assemblies loaded are formed right
above a part of the first fuel supports in the central region; the fuel assemblies
disposed in the central region are supported by the remaining first fuel supports;
and the fuel assemblies disposed in the peripheral region are supported by the second
fuel supports.
[0012] The plurality of water regions with no fuel assemblies loaded are formed right above
a part of the first fuel supports in the central region, so that the infinite neutron
multiplication factor of the fuel assemblies adjacent to the water regions can be
reduced due to the action of the cooling water in the water regions. Therefore, for
control of the excess reactivity of the initial core, the number of control rods for
control of reactor power to be inserted into the initial core can be reduced during
the operation of the reactor and the control rod operation in the reactor can be simplified.
[Advantageous Effect of the Invention]
[0013] According to the present invention, the control rod operation in the nuclear reactor
can be simplified.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
FIG. 1 is a cross sectional view showing an initial core of a nuclear reactor according
to embodiment 1 which is a preferred embodiment of the present invention.
FIG. 2 is a structural diagram showing a boiling water reactor having an initial core
shown in FIG. 1.
FIG. 3 is a structural diagram showing a fuel assembly loaded in an initial core shown
in FIG. 1.
FIG. 4 is a cross sectional view showing four fuel assemblies disposed around a neutron
detector installed in an initial core shown in FIG. 1.
FIG. 5 is a plan view showing four fuel assemblies disposed in one cell of an initial
core shown in FIG. 1.
FIG. 6 is a longitudinal cross sectional view showing a fuel support disposed in a
central portion of an initial core shown in FIG. 1.
FIG. 7 is a longitudinal cross sectional view showing a fuel support disposed in a
peripheral portion of an initial core shown in FIG. 1.
FIG. 8 is an explanatory drawing showing an insertion state of a control rod for control
of reactor power in a control cell formed in a central portion of an initial core
shown in FIG. 1.
FIG. 9 is a cross sectional view showing three different fuel assembly systems aiming
at investigation of infinite neutron multiplication factor.
FIG. 10 is an explanatory drawing showing infinite multiplication factor difference
in each fuel assembly system shown in FIG. 9.
FIG. 11 is a cross sectional view showing an initial core of a nuclear reactor according
to embodiment 2 which is another embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] The inventors executed various investigations, reduced the number of fuel assemblies
to be loaded in an initial core due to unloading of a part of the fuel assemblies,
and furthermore created a new constitution of the initial core capable of simplifying
control rod operation in the initial core. The investigation results and the outline
of the newly created initial core will be explained below.
[0016] The inventors noted that excess reactivity of the initial core is higher than that
of an equilibrium core and thus, control rod insertion amount of the initial core
is larger than that of the equilibrium core, created an idea that a water region having
a cross sectional area capable of disposing the fuel assembly in the initial core
may be disposed in the initial core. The inventors investigated the three fuel assembly
systems which are a part of the initial core shown in FIG. 9 before creating the idea
that the water region may be disposed in the initial core. A boundary 30 of the fuel
assembly systems (see FIG. 9) reflects perfectly and is an infinite system. A first
fuel assembly system is shown in (a) of FIG. 9. The first fuel assembly system shown
in (a) of FIG. 9 imitates the core operation state with no control rods inserted.
In the first fuel assembly system 9, four fuel assemblies 4 are disposed in the neighborhood
of each other. Among the four fuel assemblies 4, the mean enrichment of one fuel assembly
4A1 positioned on an upper left on the paper sheet of (a) of FIG. 9 is 2.0 wt% and
the mean enrichment of the other three fuel assemblies 4A2 is 4.0 wt%. The mean enrichment
of the four fuel assemblies 4 in the first fuel assembly system is 3.5 wt%. The mean
void fraction in the fuel assemblies 4 in the first fuel assembly system is 40% which
is a general mean void fraction of the core.
[0017] A second fuel assembly system shown in (b) of FIG. 9 includes four fuel assemblies
4 similar to the first fuel assembly system and furthermore, one control rod 5 being
adjacent to one fuel assembly 4B positioned on an upper left on the paper sheet of
(b) of FIG. 9 is inserted into the core. The respective mean enrichments of the four
fuel assemblies in the second fuel assembly system are the same as those of the first
fuel assembly system shown in (a) of FIG. 9.
[0018] In a third fuel assembly system shown in (c) of FIG. 9, no control rods are inserted,
and one fuel assembly 4 on an upper left on the paper sheet of (c) of FIG. 9 is not
loaded, and a water region 6a having a cross sectional area capable of disposing one
fuel assembly 4 is formed in the upper left position. In the third fuel assembly system,
the mean enrichment of the three fuel assemblies 4 is 4.0 wt%.
[0019] In these three fuel assembly systems, the inventors obtained the infinite neutron
multiplication factor. Assuming the void fraction in the respective fuel assemblies
in each fuel assembly system as 40%, on the basis of the infinite neutron multiplication
factor of the first fuel assembly system shown in (a) of FIG. 9, the difference between
this infinite multiplication factor and the infinite neutron multiplication factor
in each of the second and third fuel assembly systems is shown in FIG. 10. The infinite
neutron multiplication factor in the third fuel assembly system shown in (c) of FIG.
9 is lower than the infinite neutron multiplication factor in the second fuel assembly
system shown in (b) of FIG. 9 with one control rod inserted. As a result, the inventors
newly found that the water region 6a having a cross sectional area capable of disposing
one fuel assembly 4 and being formed in the third fuel assembly system shown in (c)
of FIG. 9 has a function of reducing the infinite neutron multiplication factor more
than one control rod. The infinite neutron multiplication factor of the third fuel
assembly system is lowered by about 14% Δk compared with that of the first fuel assembly
system shown in (a) of FIG. 9 and is lowered by about 9% Δk compared with that of
the second fuel assembly system shown in (b) of FIG. 9. As a result, the water region
having a cross sectional area capable of disposing one fuel assembly 4 is formed in
the initial core during the operation of the reactor, thus the excess reactivity of
the initial core can be controlled, and there is no need to insert the control rods
into the initial core during the operation of the reactor in order to control the
excess reactivity. Therefore, the inventors found that in the initial core, the formation
of the water region having a cross sectional area capable of disposing one fuel assembly
4 is effective in the control of the excess reactivity of the initial core.
[0020] The embodiments of the present invention with the aforementioned investigation results
reflected on will be explained below.
[Embodiment 1]
[0021] An initial core of a nuclear reactor according to embodiment 1 which is a preferable
embodiment of the present invention, will be explained by referring to FIG. 1.
[0022] Firstly, a rough structure of a boiling water reactor to which the initial core of
the present embodiment is applied will be explained by referring to FIGs. 1 and 2.
The boiling water reactor 1 is provided with a core 3 which is an initial core in
a reactor pressure vessel 2. The core 3 is surrounded by a cylindrical core shroud
7 installed in the reactor pressure vessel 2. A shroud head 10 covering the core 3
is installed at an upper end of the core shroud 7 and a steam separator 11 is attached
to the shroud head 10 and is extended upward. A steam dryer 12 is disposed above the
steam separator 11. The shroud head 10, the steam separator 11, and the steam dryer
12 are disposed in the reactor pressure vessel 2.
[0023] An upper lattice plate 27 is disposed in the core shroud 7 under the shroud head
10, is attached to the core shroud 7, and is positioned at the upper end of the core
3. A core support plate 8 is positioned at the lower end of the core 3, is disposed
in the core shroud 7, and is installed in the core shroud 7. A plurality of internal
pumps 13 are attached to bottom of the reactor pressure vessel 2 and an impeller of
each internal pump 13 is disposed in an annular down comer 14 formed between the core
shroud 7 and the reactor pressure vessel 2. A plurality of fuel supports 9 are installed
on the core support plate 8. A plurality of control rod guide pipes 15 are disposed
in the reactor pressure vessel 2 under the core support plate 8. Control rods 5 having
a cross-shaped cross section are respectively disposed in the respective control rod
guide pipes 15 and each control rod 5 is connected to a control rod drive mechanism
16 installed in a control rod driver housing (not drawn) attached to the bottom of
the reactor pressure vessel 2.
[0024] A plurality (for example, 872 each) of fuel assemblies 4 are loaded in the core 3.
The burnup of all the fuel assemblies loaded in the core 3 which is the initial core
is 0 GWd/t before start of the operation of the boiling water reactor including the
core 3. The boiling water reactor 1 including the core 3 with 872 fuel assemblies
4 loaded uses 205 control rods 5.
[0025] The fuel assembly 4 loaded in the core 3 will be explained by referring to FIG. 3.
The fuel assembly 4 has a plurality of fuel rods 20, an upper tie plate 23, a lower
tie plate 24, and a channel box 22. Many columnar fuel pellets manufactured using
a nuclear fuel material including a fissional material (uranium 235) are filled in
the fuel rods 20. A lower end portion of each fuel rod 20 is supported by the lower
tie plate 24 and an upper end portion of each fuel rod 20 is held by the upper tie
plate 23 with a handle 23a installed. The respective fuel rods 20 are arranged in
a square lattice shape (see FIG. 4) and are bundled by a plurality of fuel spacers
25 so as to hold predetermined intervals between the mutual fuel rods. A plurality
of fuel spacers 25 are disposed in an axial direction of the fuel assembly 4. Partial
fuel rods 20A are disposed in an inner layer adjacent to an outermost layer of the
arrangement of the fuel rods 20 as shown in FIG. 4. In a central portion of the cross
section of the fuel assembly 4, two water rods 21 are disposed adjacently to each
other and the respective fuel rods 20 surround the periphery of the water rods 21
(see FIG. 4). Also in the water rods 21, a lower end portion is supported by the lower
tie plate 24 and an upper end portion is held by the upper tie plate 23. A plurality
of fuel rods 20 and water rods 21, which are bundled by the plurality of fuel spacers
25, are disposed in the channel box 22, an upper end portion of which is attached
to the upper tie plate 23 and is extended toward the lower tie plate 24. In a part
of the fuel rods 20 in the fuel assembly 4, the fuel pellets include a burnable poison.
In FIG. 4, numeral 28 indicates a neutron detector arranged in the core 3.
[0026] As shown in FIG. 5, in a state that upper end portions of the four fuel assemblies
4 are inserted in the respective square spaces formed in the upper lattice plate 27,
the upper end portions of the four fuel assemblies 4 are pressed to the upper lattice
plate 27 by a channel fastener 26 attached to the upper end of the channel box 22
of each fuel assembly 4 and held by the upper lattice plate 27. These four fuel assemblies
4 are disposed in the neighborhood of one control rod 5 and surround the control rod
5. One cell is formed by the one control rod 5 and the four fuel assemblies 4 disposed
in the neighborhood of the control rod. The core 3 includes a plurality of cells.
[0027] A plurality of fuel supports 9 removably attached to the core support plate 8 include
a plurality of fuel supports 9a (see FIG. 6) disposed in a central portion 3a of the
core 3 and a plurality of fuel supports 9b (see FIG. 7) disposed in a peripheral portion
3b of the core 3. The central portion 3a is surrounded by the peripheral portion 3b.
Since the fuel supports 9a and the fuel supports 9b have substantially the same structure,
the outline of the structure of the fuel support 9 will be explained using the fuel
support 9a as an example. The diameter of the central portion 3a is 16/17 of the diameter
of the core 3. The fuel support 9a includes a support body 29. In the support body
29, a through hole 31 having a cross-shaped cross section extending on all sides from
an axial center for inserting the control rod 5 is formed and four cooling water supply
passages 32 disposed so as to surround the through hole 31 are formed. One end of
each cooling water supply passage 32 is opened at an upper end of the support body
29 and other end of each cooling water supply passage 32 is opened on side of the
support body 29. An orifice 33a is installed in each cooling water supply passage
32 in the neighborhood of the side of the support body 29.
[0028] The fuel support 9b also has the aforementioned structure of the fuel support 9a.
However, the fuel support 9a and the fuel support 9b are different from each other
in one point. It is that opening area of the orifice 33a installed in the fuel support
9a is smaller than opening area of an orifice 33b installed in the cooling water supply
passage 32 of the fuel support 9b. In other words, it is that pressure loss of the
orifice 33a of the fuel support 9a is larger than pressure loss of the orifice 33b
of the fuel support 9b.
[0029] A lower end portion 24a of each lower tie plate 24 of the four fuel assemblies 4
in each cell existing in the central portion 3a of the core 3 is separately inserted
into the opening of each coolant supply passage 32 formed at the upper end of the
support body 29 of the fuel support 9a. In this way, each fuel assembly 4 disposed
in the central portion 3a of the core 3 is supported by the fuel support 9a. A lower
end portion 24a of each lower tie plate 24 of the four fuel assemblies 4 in each cell
existing in the peripheral portion 3b of the core 3 is separately inserted into the
opening of each coolant supply passage 32 formed at the upper end of the support body
29 of the fuel support 9b. In this way, each fuel assembly 4 disposed in the peripheral
portion 3b of the core 3 is supported by the fuel support 9b.
[0030] Four control cells 34 are disposed in the central portion 3a of the core 3 where
the fuel supports 9a are disposed. These control cells 34 are disposed in a position
at an equal distance from an axial center of the core 3. Furthermore, five water regions
6 are disposed in the central portion 3a. One water region 6 is disposed at the axial
center of the core 3 and other four water regions 6 are disposed in a position at
an equal distance from the axial center of the core 3. These four water regions 6
are respectively positioned in the square corners surrounding the axial center of
the core 3 on the cross section of the core 3. Each control cell 34 is disposed at
the position of the middle point of each side of the square connecting the two water
regions 6. The positions where the five water regions 6 are disposed are the positions
where the control cells 34 are disposed in the conventional initial core.
[0031] Each control cell 34 is a cell of the reactor, into which the control rod 5 for reactor
power control for controlling the reactor power is inserted during the rated operation
of reactor power of 100%. The infinite multiplication factor of the four fuel assemblies
4 in each control cell 34 is lower than the infinite multiplication factor of the
fuel assembly in the cell other than the control cell 34, the cell existing around
the control cell 34, in the central portion 3a.
[0032] The five water regions 6 are regions having a square cross section for occupying
the cross sectional area capable of disposing four fuel assemblies 4. These water
regions 6 are a space formed between the fuel assemblies 4 and having a square cross
section for occupying the cross sectional area capable of disposing four fuel assemblies
before the construction of the boiling water reactor including the initial core of
the present embodiment is finished and cooling water is filled in the reactor pressure
vessel 2. When cooling water is filled in the reactor pressure vessel 2, the cooling
water is filled also in the space and the water regions 6 are formed. After cooling
water is filled in the reactor pressure vessel 2, cooling water exists in the five
water regions 6. In each water region 6, no fuel assemblies exist.
[0033] After start of the operation of the boiling water reactor, each control rod 5 is
withdrawn from the core 3 and the boiling water reactor reach a state of criticality
from a state of subcriticality. The withdrawal operation of each control rod 5 and
the insertion operation which will be described later are performed by the control
rod drive mechanism 16. Furthermore, when each control rod 5 inserted in the core
3 is gradually withdrawn, the reactor power is increased. When the reactor power becomes,
for example, about 60% by the withdrawal of the control rods 5, the withdrawal of
the control rods 5 is stopped. Thereafter, the number of revolutions of each internal
pump 13 is increased, and the cooling water flow rate supplied to the core 3 is increased.
Thus, the core flow rate is increased, and the reactor power is increased up to the
rated power (100%). When the reactor power reaches the rated power, the increase of
the core flow rate is stopped.
[0034] At this time, in the central portion 3a of the core 3, all the control rods 5 disposed
in all the cells other than the control cells 34 and in all the water regions 6 are
withdrawn from the core 3. In the central portion 3a, the upper end of each handle
of all the control rods 5 withdrawn from the core 3 is positioned under the upper
end of the fuel support 9a as shown in FIG. 6. Also in the peripheral portion 3b of
the core 3, all the control rods 5 disposed in all the cells are withdrawn from the
core 3. In the peripheral portion 3b, the upper end of each handle of all the control
rods 5 withdrawn from the core 3 is also positioned under the upper end of the fuel
support 9b as shown in FIG. 7. In the four control cells 34 in the central portion
3a, the control rod 5 for reactor power control is inserted into the core 3 and as
shown in FIG. 8, the upper end of the control rod 5 is positioned above the upper
end of the fuel support 9a.
[0035] The cooling water in the down comer 14 is pressurized by the drive of the internal
pumps 13 and is supplied to the core 3 through a lower plenum 17 formed under the
core 3. Concretely, the greater part of cooling water reaching the lower plenum 17
is supplied into the respective fuel assemblies 4 supported by the fuel supports 9a
through each cooling water supply passage 32 of the fuel support 9a and furthermore
is supplied into the respective fuel assemblies 4 supported by the fuel support 9b
through each cooling water supply passage 32 of the fuel support 9b. The cooling water
rising in the channel box 22 of each fuel assembly 4 is heated by heat generated by
the nuclear fission of a fissional material filled in the fuel rods 20 and the partial
fuel rods 20A and a part of the heated cooling water is vaporized. The gas-liquid
two-phase flow including steam and cooling water is discharged above the core 3 through
a through hole (not drawn) formed in the upper tie plate 23 of each fuel assembly
4.
[0036] The remaining cooling water reaching the lower plenum 17 is introduced into the respective
control rod guide pipes 15 through an opening (not shown) formed in each control rod
guide pipe 15. The cooling water is supplied to water gaps 35 formed between the fuel
assemblies 4 being adjacent to each other through the through holes 31 formed in the
fuel supports 9a and 9b. The cooling water goes up in each water gap 35. Even in the
fuel supports 9a existing right under the water regions 6, the cooling water flowing
into the control rod guide pipes 15 is introduced into the through holes 31. This
cooling water is supplied into the water regions 6 from the through holes 31. The
respective cooling water supplied into each water region 6 and each water gap 35 is
heated by heat discharged from the inside of the fuel assemblies 4 and goes up in
each water region 6 and each water gap 35. However, the cooling water flowing in each
water region 6 and each water gap 35 does not boil. In the respective fuel supports
9a existing right under each water region 6, the entrance of each cooling water supply
passage 32 formed in these fuel supports 9a is blocked so as to prevent the cooling
water from being supplied to the water regions 6 through the cooling water supply
passages 32.
[0037] The cooling water rising in each water region 6 and each water gap 35 is discharged
from each water region 6 and each water gap 35 and is mixed with the gas-liquid two-phase
flow discharged from each fuel assembly 4. The gas-liquid two-phase flow including
the cooling water discharged from the water regions 6 and the water gaps 35 is led
into the steam separator 11. The steam included in the gas-liquid two-phase flow is
separated from the cooling water by the steam separator 11 and is introduced to the
steam dryer 12. The steam from which moisture is further removed by the steam dryer
12 is supplied to a turbine (not drawn) through a main steam pipe 18. The turbine
is rotated by the steam and rotates a generator (not drawn) connected to the turbine.
Electricity is generated by the rotation of the generator. Steam discharged from the
turbine is condensed by a condenser (not drawn) to water. This water is supplied into
the reactor pressure vessel 2 as feed water through a feed water pipe 19.
[0038] The cooling water separated from the gas-liquid two-phase flow by the steam separator
11 is introduced into the down comer 14 and is mixed with feed water supplied from
the feed water pipe 19 in the down comer 14. This cooling water is pressurized by
the internal pumps 13 and as described before, is supplied into the core 3, that is,
each fuel assembly 4.
[0039] When the reactor power is lowered than the rated power in correspondence to the progress
of the operation in the first cycle of the boiling water reactor 1, the core flow
rate is increased and the reactor power is kept at the rated power. However, when
the core flow rate increases to 100%, the core flow rate is reduced, and the reactor
power is reduced down to a predetermined reactor power lower than about 60%, and then
the control rod pattern is exchanged. Due to the exchange of the control pattern,
the control rods 5 in the control cells 34 are withdrawn and the reactor power is
increased up to about 60%. Thereafter, the core flow rate is increased and the reactor
power is increased up to the rated power. The reduction of the reactor power from
the rated power in correspondence to consumption of the fissional material is compensated
by an increase in the core flow rate. When the core flow rate increases again to 100%,
as mentioned above, the control rod pattern is exchanged. The control rod pattern
exchange is repeated until all the control rods 5 in the control cells 34 are withdrawn.
When all the control rods 5 in the control cells 34 are withdrawn and the core flow
rate increases to 100%, the operation of the boiling water reactor 1 in the first
cycle is finished. At the time, all the control rods 5 are inserted into the core
3, and the boiling water reactor 1 is stopped.
[0040] In the present embodiment, when the boiling water reactor 1 is in operation, the
control of the excess reactivity of the reactor in the first cycle of the initial
core is executed by the cooling water in the water regions 6 using the cooling water
in the water regions 6 in addition to neutron absorber (for example, B
4C) included in the control rods 5 in the control cells 34 and burnable poison included
in the fuel assemblies 4. As described before, the infinite neutron multiplication
factor of the fuel assemblies 4 is reduced by each water region 6 in which cooling
water exists, so that the number of control cells can be reduced than that in the
conventional initial core and in correspondence to it, the number of the control rods
5 for reactor power control can be reduced. Therefore, in correspondence to the reduction
in the number of the control rods 5 for reactor power control, the withdrawal operation
of the control rods 5 for reactor power control in the control cells 34 during the
rated power operation of the boiling water reactor 1 can be simplified.
[0041] In the present embodiment, the cells of the water regions 6 in the first cycle are
changed to the control cells 34 in the second cycle, so that the cells changing the
water regions 6 to the control cells 34 can lengthen the life span of the control
rods 5. Namely, in the first cycle, each control rod 5 disposed in the position of
each water region 6 is in a state that all the control rods are withdrawn from the
core 3 during a period of the operation in the first cycle. Therefore, in the second
cycle, substantially fresh control rods 5 are inserted into the cells changed to the
control cells 34 from the water regions 6.
[0042] In the present embodiment, in the first cycle, the fuel assemblies 4 are not disposed
in the respective water regions 6, so that the number of spent fuel assemblies taken
out from the core 3 after end of the first cycle can be reduced and the fuel cycle
cost of the initial fuel can be reduced.
[Embodiment 2]
[0043] An initial core of a nuclear reactor according to embodiment 2, which is another
embodiment of the present invention, will be explained by referring to FIG. 11.
[0044] A core 3A which is the initial core of the present embodiment has a structure that
in the core 3 of the embodiment 1, the water region 6 disposed at the axial center
of the core is removed, and two water regions 6B having a square cross section for
occupying the cross sectional area capable of disposing one fuel assembly 4 are disposed
in each cell in the diagonal direction instead of the water regions 6 which are regions
having a square cross section for occupying the cross sectional area capable of disposing
four fuel assemblies 4. The other structure of the core 3A is the same as that of
the core 3.
[0045] In each cell that the two water regions 6B are disposed in the diagonal direction,
two fuel assemblies 4 are disposed in another diagonal direction orthogonal to the
diagonal line connecting the two water regions 6B. In the core 3A, eight cells that
two water regions 6B are disposed in the diagonal direction are formed. The water
region 6 disposed at the center of the core 3A forms a region having a square cross
section for occupying the cross sectional area capable of disposing four fuel assemblies
4 similarly to the water region 6 in the embodiment 1. The control cells 34 are respectively
disposed at a middle point of each side of a square surrounding the water region 6,
in which four cells with two water regions 6B disposed in the diagonal direction are
disposed in the corners. The diameter of the central portion 3a is 16/17 of the diameter
of the core 3A.
[0046] At the time of the rated power operation of the boiling water reactor including the
core 3A, the control rods 5 of each cell that two water regions 6B having a square
cross section for occupying the cross sectional area capable of disposing one fuel
assembly 4 are disposed in the diagonal direction are in a state that all the control
rods 5 are withdrawn from the core 3A.
[0047] The present embodiment can obtain the effects generated in the embodiment 1. Furthermore,
in the present embodiment, the number of water regions 6 having a square cross section
for occupying the cross sectional area capable of disposing four fuel assembly 4 is
smaller than that of the core 3 and instead, a plurality of cells that two water regions
6B are disposed in the diagonal direction are formed in the core 3A, so that the power
distribution in a radius direction of the core is more flattened than that of the
embodiment 1 and economical efficiency of fuel can be improved.
[0048] In the boiling water reactor including the core 3 or 3A, a method of loading fuel
assemblis of a nuclear reactor described below can be applied. The method of loading
fuel assemblies that is applied to the core 3 will be explained.
[0049] In the boiling water reactor 1 having the core 3 which is an initial core, a plurality
of fuel assemblies 4 having a burnup of 0 GWd/t are loaded in the core 3 before starting
the operation of the boiling water reactor 1. These fuel assemblies 4 are successively
loaded in the region other than the region for forming the water regions 6 in the
core 3. The fuel assemblies 4 having a burnup of 0 GWd/t are not loaded in the region
for forming the water regions 6. The boiling water reactor 1 having the core 3 formed
by loading these fuel assemblies 4 is operated in the first cycle which is a first
operation cycle after the boiling water reactor 1 is constructed. The operation in
the first cycle is finished and the boiling water reactor 1 is stopped. After the
stop of the boiling water reactor 1, a part of the fuel assemblies 4 in the core 3
is taken out from the reactor pressure vessel 2 and is exchanged with fresh fuel assemblies
4. Spent fuel assemblies 4 taken out from the reactor pressure vessel 2 for fuel exchange
are fuel assemblies 4 having a low mean enrichment which are loaded in the core 3
before start of the first cycle. The fuel assemblies 4 having a high mean enrichment
which are loaded in the core 3 before start of the first cycle are not taken out from
the reactor pressure vessel 2 after end of the operation in the first cycle and exist
in the core 3 even at the time of the operation in the next second cycle.
[0050] When the operation in the first cycle is finished and the fuel assemblies 4 in the
core 3 are exchanged, a part of the fuel assemblies 4 having a low mean enrichment
which are loaded in the core before start of the first cycle, are loaded in the water
regions 6, respectively. In each of all the water regions 6, four fuel assemblies
4 having a low mean enrichment which are loaded in the core before start of the first
cycle are loaded.
[0051] Fresh fuel assemblies 4 having a high mean enrichment and a burnup of 0 GWd/t are
loaded in the position in the core 3 where the fuel assemblies 4 taken out from the
reactor pressure vessel 2 exist during the operation in the first cycle and in the
position in the core 3 where the fuel assemblies 4 loaded in the water regions 6 exist
during the operation in the first cycle, respectively.
[0052] After the loading of the fuel assemblies 4 aforementioned is finished, the operation
of the boiling water reactor 1 in the second cycle is started. The four water regions
6 formed in the core 3 in the first cycle become control cells 34 so that four fuel
assemblies 4 having a lower infinite neutron multiplication factor than that of the
fuel assemblies 4 surrounding the control cell 34 are loaded and a control rod 5 for
reactor power control is disposed, in the second cycle. In the second cycle, the number
of control cells 34 is increased than that in the first cycle. In the second cycle,
the water regions 6 substituted for the control rods 5 in the first cycle do not exist,
so that the insertion amount of the control rods 5 is increased and the number of
control cells (cells for adjusting the reactivity by the control rods) 34 is increased.
[0053] Also to the boiling water reactor having the core 3A, the method of loading fuel
assemblies applied to the boiling water reactor having the core 3 can be applied.
[REFERENCE SIGNS LIST]
[0054]
1 : boiling water reactor, 2 : reactor pressure vessel, 3, 3A : core, 4 : fuel assembly,
5 : control rod, 6, 6a, 6b : water region, 7 : core shroud, 8 : core support plate,
9, 9a, 9b : fuel support, 13 : internal pump, 15 : control rod guide pipe, 16 : control
rod drive mechanism, 20 : fuel rod, 22 : channel box, 29 : support body, 31 : through
hole, 32 : cooling water supply passage, 33a, 33b : orifice, 35 : water gap.